1. Field of the Invention
The present invention relates to the profiling of geological formations. More particularly, the present invention relates to the use of a drilling tool as a frequency-controlled seismic source for the inverse or reverse vertical seismic profiling or imaging of a formation.
2. Description of the Related Art
Conventional reflection seismology utilizes surface sources and surface receivers to detect reflections from subsurface acoustic impedance contrasts; i.e., subsurface strata. The image and information obtained utilizing conventional reflection seismology is considered relatively poor due to long travel paths between the sources, reflectors, and receivers.
A technique known as vertical seismic profiling (VSP) has been used to provide more accurate information regarding the formation in the vicinity of a borehole. In VSP, seismic sources are located at the surface, and sensors (geophones) are suspended in the borehole with a wireline. When the sources are fired, the received data is recorded and processed for several borehole depths. VSP has the drawbacks, however, that each surface source is costly to apply, adds to acquisition time and rig inactivity, and generates multiple ghost images due to energy trapped in the surface layer. In addition, in drilling applications where the geophones would have to be added to an MWD (measurement while drilling) tool string, unless the downhole geophones could be decoupled from the drill string, they would only be capable of recording the relatively large first arrival event. Furthermore, downhole geophones are sensitive to tube wave events traveling up and down the borehole. Thus, to date, vertical seismic profiling has not been successful in MWD applications.
In order to overcome some of the problems relating to explosive sources used in conjunction with VSP, "Vibroseis" (a trademark of Conoco Incorporated) sources have been utilized. The "Vibroseis" source (described in U.S. Pat. No. 2,688,124) comprises vibrators which are carried on trucks which are coupled to the formation surface and which generate a frequency sweep from 10-90 Hz over a period of time. The trucks move to pre-designated source locations, lower their heavy baseplates, and start vibrating through the use of a hydraulic system. However, the "Vibroseis" source technique still suffers from several drawbacks. These include difficulty in correctly locating the trucks and in establishing good ground contact.
Long ago it was recognized that it might be possible to switch the location of the seismic source and the detectors; i.e., provide an "inverse" or "reverse" VSP (hereinafter referred to as "reverse VSP" or "RVSP"). A downhole source overcomes the logistical limitations inherent in using multiple surface sources, and permits simultaneous data acquisition at the surface with little cost.
Various different downhole source schemes have been proposed over the years. Some of the downhole source schemes utilize the drill bit as the source, while others use sources other than the drill bit. Of the former, U.S. Pat. No. 2,062,151 to Weatherby suggests using the drill bit as an impulse generator of seismic waves, while U.S. Pat. Nos. 4,363,112 and 4,365,322 to Widrow suggest using the continuous natural random vibrations of the drill bit to launch seismic waves. Techniques such as disclosed in U.S. Pat. No. 5,050,130 to Rector et al., can be useful in processing the information obtained from such a drill bit seismic source. Of the schemes which suggest using a source other than a drill bit, U.S. Pat. No. 3,909,776 to Broding et al. suggests using a fluid driven oscillator which changes emitted frequency as a function of time (much like a "Vibroseis" source), U.S. Pat. No. 3,881,168 to Farr et al. suggests using a monofrequency fluid oscillator, while U.S. Pat. No. 4,207,619 to Klaveness suggests using a pulse generator located in the drill string just above the drill bit. In addition, U.S. Pat. No. 4,033,429 to Farr suggests providing a seismic source in a drill string by modifying a section of a drill pipe to reduce it in diameter and provide a port, and providing a cylindrical sleeve surrounding the modified drill pipe section. The sleeve is provided with mud ports arranged in a helical path, with the spacing of the ports varying from a fifteen degree spacing at one end of the helix to a one hundred twenty degree spacing at the other end of the helix. To create a seismic signal, the entire drill string is lifted off of the hole bottom, and the drill pipe (and drill bit) is rotated and moves downward. The movement of the drill pipe port past the sleeve ports generates a series of pressure pulses in a siren-like fashion which change in frequency due to the spacing of the sleeve ports. The rotation rate of the drill pipe will therefore control the specific frequency range of the sweep.
While the idea of utilizing a "Vibroseis" type seismic source downhole is conceptually advantageous, the techniques proposed in the art for generating such a source have not been feasible. In particular, the fluid driven oscillator of Broding et al. will not provide a discrete point of signal excitation and will thereby "smear" the seismic image, and the oscillator must be inconveniently added to the drill string at extra expense. Additionally, in deviated wells, wellbore friction makes smooth rotation of the drill string impossible, causing variations in signal frequency. Finally, the drill string must be removed from the borehole each time the system is operated, forcing costly delays in the drilling process. With respect to U.S. Pat. No. 4,033,429 to Farr, the requirement to modify and rotate the drill pipe in order to effect a seismic source is disadvantageous for several reasons. First, in Farr, the drilling process and the seismic signal generation process are exclusive of each other, such that tripping out is required to move from drilling to seismic signal generation and vice versa. Second, the rotation of the drill pipe during seismic signal generation will cause banging on the side of the well which will generate undesirable noise. Third, because the seismic signal is being generated by a pressure differential across different orifices, the location where the pressure is being generated (i.e., the source signal location) will be undesirably uncertain.